Non-insulin-dependent diabetes mellitus (NIDDM) is preceded by impaired glucose tolerance (IGT) lasting for years before manifesting as overt hyperglycaemia. Both genetic and environmental factors contribute to the development of IGT and NIDDM. Obesity, physical inactivity and high-fat diet have been found to predict IGT and NIDDM. Therefore, a diet and exercise intervention from diagnosis of NIDDM could improve the treatment outcome and prognosis of patients with NIDDM. Furthermore, because subjects with IGT are at increased risk for diabetes and atherosclerotic vascular disease, it is reasonable to assume that in terms of reducing the incidence and longterm consequences of NIDDM an intervention at this phase is more effective than in overt diabetes. Although the nonpharmacological approach is generally accepted as the first-line treatment of NIDDM its efficacy has often been questioned. Therefore, it is important to carry out long-term controlled studies to find out to what extent lifestyle modification could improve the metabolic control and level of major cardiovascular risk factors known to be associated with poor outcome in NIDDM. This kind of study also gave relevant experience in planning studies aiming at primary prevention of NIDDM. One-year dietary and exercise intervention on newly diagnosed NIDDM patients in Kuopio, Finland resulted in a better metabolic control and a moderate reduction in cardiovascular risk factors as compared to the conventional treatment group. After the second year of follow-up only 12.5% in the intervention group were receiving oral antidiabetic drugs vs. 34.8% in the conventional treatment group. Weight reduction and a reduced use of saturated fats appeared to be the main determinants of successful treatment results. Good aerobic capacity was associated with an increase in HDL cholesterol. A multicentre primary prevention study on IGT patients is continuing in Finland applying the same principles of intervention as used in the study on newly diagnosed NIDDM patients. Pilot results show that glucose tolerance can be improved by lifestyle changes.